Litter and its impact on our environment is receiving increased national attention. In response to this problem, some states have enacted legislation which has banned the use of throw-away plastic, glass, and metal pop bottles. Recent studies have shown that this legislation has reduced the amount of litter on our highways and other public property. However, this legislation has also created other unforseen problems in the storage and return of returnable pop bottles.
It is common practice in the beverage industry to use plastic receptacles for shipping glass, plastic and metal pop containers. These plastic receptacles are normally formed with a plurality of stiffening flanges and ribs which form a large number of crevices within which dirt can accumulate. In some instances, the plastic receptacles are composite structures which include removable secondary closures. For hygenic and aesthetic purposes, it is desirable to clean these receptacles or structures on each occasion when they are returned to the bottling plant.
The art of cleaning an object by dipping it in an acidic or basic solution so that the chemical solution attacks the surface contaminate is well known. This method is economical, requiring simple equipment. However, it is also time consuming and does not always clean the crevices and the holes in the objects being so processed.
Cleaning objects by placing them in a liquid bath and transmitting ultrasonic waves through the bath to impinge against them is also well known. A piezoelectric transducer is excited by a radio frequency generator at a frequency substantially above that of ordinary sound. One face of the transducer is in engagement with the cleaning fluid and sets up alternate compressions and rarification waves in the bath at the excitation frequency. These waves in turn create cavitation at the surface of the object to be cleaned which results in a gentle scrubbing action to remove surface contaminates. The cavitation is the result of a formation of bubbles within the cleaning fluid wherever there are imperfections such as microscopic nuclei or absorbed air therein. These bubbles expand in the rarification or the tension portion of the energy wave and contract during the compression portion. If the size after expansion exceeds a critical ratio to the initial size, the bubble will burst and "crash" thus producing the scrubbing action at the surface of the material to be cleaned. By using ultrasonic cleaning in a suitable cleaning fluid or detergent, objects having irregularly shaped surfaces and crevices can be cleaned very rapidly. Certain types of surface contaminates, which are removed with difficulty or not at all by other cleaning processes may be readily removed by ultrasonic cleaners. In some cases, however, the amount of ultrasonic equipment required to clean the surface of an object in a given time is large and, therefore, expensive. In addition, the extent to which the objects may be contaminated with dirt varies quite substantially. In some instances, a great deal of dirt and dust may have accumulated in the crevices and it has been found that great difficulty has been experienced in attempting to clean such objects by the previously known prior art ultrasonic cleaning devices.
It has, therefore, been suggested that the ultrasonic cleaning apparatus be combined with a chemical cleaning apparatus to improve the speed and efficiency of the chemical cleaning apparatus and to reduce the cost of ultrasonic cleaning. However, the combining of these two methods has given rise to additional problems. One problem encountered in known prior art designs is that the chemical wash chamber may have an undesirable attenuating effect on the ultrasonic energy produced by the transducer. Because of such attenuation, greater amounts of transducer input power are required for a given cleaning effect. An increase in transducer input power causes increased cavitation at the surface of the transducer but it reduces the transducer's useful life. Another problem encountered by combining these two cleaning systems is that none of the known prior art designs have a mechanism by which the ultrasonic cleaning bath can be bypassed after the object has been cleaned in a wash chamber. One prior art design combining the chemical cleaning with an ultrasonic apparatus is disclosed in U.S. Pat. No. 4,170,241 to Clapp. This design first loosens the dirt on the object by passing the object through the ultrasonic washer. The object is then passed through two stages of rinsing in order to mechanically remove the loosened dirt from the object. This apparatus, however, is designed to clean dirty objects more by repetitively passing the object through the apparatus rather than being designed to remove all accumulated contamination at one time. Another example of such a prior art device is shown in Russian Pat. No. 2,828,887. This apparatus is concerned with a highly specific apparatus for cleaning machine parts. This apparatus is designed to remove the machine parts from the ultrasonic bath without at the same time collecting the oily residues on the surface of the bath.